The present invention relates to radio frequency measurement devices, and more particularly to a frequency domain reflection measurement device for detecting impedance changes and locations as well as frequency characteristics in a communications system.
Radio frequency (RF) communications components, particularly antennas and feedlines, may be improperly installed or may degrade with time. These degradations exhibit impedances that are different from an originally specified characteristic impedance of a transmission system. Measurement of these different impedances, either voltage standing wave ratio (VSWR) or return loss, and measurements of the distance from a measurement point to a point of erroneous impedance are needed before repair may begin. Identification of small degradations that have not yet caused catastrophic failure allows for preventative maintenance. In an ideal transmission line system the load of the system matches the characteristic impedance of the system, and a test signal injected at one end is absorbed completely by the load at the other end. However if there are impedance mismatches in the system, these mismatches cause a portion of the test signal to be reflected back to the source as a return signal. The amount of the magnitude of the return signal below the magnitude of the injected test signal is termed xe2x80x9creturn loss.xe2x80x9d
For example, in cellular telephone installations it is often desired to determine if transmission lines to the antenna plus the antennas themselves are undamaged. Today this test is performed using miniature, hand-held vector network analyzers (VNAs) to measure RF characteristics of the lines and antennas. Such a VNA is the Anritsu/Wiltron Sitemaster analog swept measurement device, as shown in FIG. 1. A swept frequency signal from an RF source is used to determine distance and type of problem by looking at the amplitude and phase of the return to generate a xe2x80x9cgoodnessxe2x80x9d plot of the system under test within a frequency range of interest, as shown in FIG. 2, and by using an inverse Fast Fourier Transform (FFTxe2x88x921) to convert to the time domain to determine distance. In order to increase the accuracy of detecting a fault and to increase the resolution of the distance to the fault the sweep of the swept frequency is increased over a wider range than just the frequency range of interest.
What is desired is a measurement instrument for measuring and locating changes in impedance in a transmission system which has a high dynamic range and provides accurate measurements.
Accordingly the present invention provides a frequency domain measurement device that generates a spread spectrum RF signal as a test signal that is transmitted to a device under test. A corresponding return signal from the device under test due to impedance mismatches or faults which cause reflections is correlated with the spread spectrum signal to obtain the location of and distance to the faults. A broadband signal source, such as a pseudo random number generator, is used to modulate an RF signal to generate the spread spectrum RF signal. A delayed pseudo random number generator or the equivalent may be used to correlate with the return signal, the amount of delay between transmit signal correlation and return signal correlation being indicative of the distance to an impedance mismatch. The delayed pseudo random number generator may be accomplished with a FIR filter in an error minimization loop, with the tap values of the FIR filter being processed to produce both a frequency characteristic for the device under test as well as the location of and distance to the impedance mismatches.